Downhole tools or tool strings are commonly deployed in oil and gas wells for a variety of reasons, for example to perform a well operation such as a remedial operation and/or to perform downhole measurements. It is known to lower or run a downhole tool for such purposes into position within a wellbore on the end of a support member or line and/or to recover the downhole tool to surface by hauling in the support member or line.
Some downhole tools require power to perform or enhance their function. Power may be provided in mechanical, electrical, magnetic and/or chemical form. For example, electrical power may be provided to a downhole tool either by transmission along an electrically conductive wireline or from a downhole battery.
In addition to being used for the transmission of electrical power from surface to a downhole tool, electrically conductive wirelines may also be used for electrical communications between the downhole tool and surface, for example for the electrical transmission of well logging data to surface. Electrically conductive wirelines generally have a steel wire outer armour consisting of one or more layers of helically twisted steel wires around an electrically insulated core of one or more electrical conductors. Such conventional wirelines present a sealing hazard against the well pressure at surface since gas pressure may migrate in the interstitial voids of the armour. Accordingly, wireline operations are generally costly and involve a surface sealing safety risk.
Mechanical power can be delivered by steel cables such as swabbing lines or slicklines. Slicklines have a smooth outer surface against which well pressure sealing at surface can be simply and safely performed by stuffing box sealing glands. Slicklines have conventionally been used to mechanically support and transport downhole tools. In addition, slicklines have been used to transfer mechanical power to downhole tools from a winch located at surface. However, slicklines are not suitable for the transfer of electric power. Accordingly, downhole tools which require electric power but which are configured for use with slickline are generally provided with their own batteries.
Downhole conditions are hostile to battery performance. Thus, it is generally necessary to protect batteries from hostile downhole conditions by housing the batteries in sealed enclosures. This limits the available space for the batteries and thus limits the power available downhole. The size and shape of the battery enclosure is generally constrained by the geometry of a lubricator located at the wellhead and the inside diameter of the casing tubular through which the downhole tool must pass. Furthermore, the high downhole temperatures limit the electrical power storage and output capacity of the batteries. To safeguard downhole operations, it is also important to be able to monitor the battery performance and control the consumption of battery power.
In deviated oil and gas wells, it may not be possible to lower a downhole tool to a desired position. This is particularly true in highly deviated oil and gas wells where a deviated section of the wellbore may extend in a horizontal or near horizontal direction. Accordingly, it is known to use a downhole tractor to advance a downhole tool along a deviated section of an oil and gas wells. Conventional downhole tractors are typically either wheel-driven or are of the reciprocating or inchworm type. Wheel-driven tractors generally have wheels mounted on powered pivot arms which are pressed against the tubing inner walls. Reciprocating tractors generally include a forward body having forward clamp shoes and a rear body having rear clamp shoes. The forward and rear bodies are configured to reciprocate relative to one another. The forward and rear clamp shoes are alternately pressed against the inner wall of a tubular or a wellbore. The forward body is pushed in the downhole direction relative to the rear body against the rear clamp shoe or the rear body is pulled relative to the forward body against the forward clamp shoe to advance the rear body.
Known downhole tractors may be supplied with electric power from surface via a wireline or are provided with batteries for the supply of power to the tractor drive arrangement. For example, US 2010/0263856 discloses a battery-driven downhole tractor which is run on a conventional slickline. Alternatively, it is known to supply mechanical power to a downhole tractor from surface. For example, WO 99/24691 discloses a downhole tractor suspended from a wireline which may be electricline, slickline or any other wire or tubular system which is capable of reciprocating movement. The tractor is run into a wellbore until the tractor encounters a deviated section of the wellbore. The tractor is advanced along the deviated section of the wellbore by the repeated application and release of tension in the wireline. Advancing a tractor in this way is a manually intensive time-consuming process which may result in relatively high operating costs.